accept(2) System Calls Manual accept(2)
NAME
accept, accept4 - accept a connection on a socket
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/socket.h>
int accept(int sockfd, struct sockaddr *_Nullable restrict addr,
socklen_t *_Nullable restrict addrlen);
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sys/socket.h>
int accept4(int sockfd, struct sockaddr *_Nullable restrict addr,
socklen_t *_Nullable restrict addrlen, int flags);
DESCRIPTION
The accept() system call is used with connection-based socket types (SOCK_STREAM, SOCK_SEQPACKET). It extracts the first
connection request on the queue of pending connections for the listening socket, sockfd, creates a new connected socket, and
returns a new file descriptor referring to that socket. The newly created socket is not in the listening state. The origi‐
nal socket sockfd is unaffected by this call.
The argument sockfd is a socket that has been created with socket(2), bound to a local address with bind(2), and is listen‐
ing for connections after a listen(2).
The argument addr is a pointer to a sockaddr structure. This structure is filled in with the address of the peer socket, as
known to the communications layer. The exact format of the address returned addr is determined by the socket's address fam‐
ily (see socket(2) and the respective protocol man pages). When addr is NULL, nothing is filled in; in this case, addrlen
is not used, and should also be NULL.
The addrlen argument is a value-result argument: the caller must initialize it to contain the size (in bytes) of the struc‐
ture pointed to by addr; on return it will contain the actual size of the peer address.
The returned address is truncated if the buffer provided is too small; in this case, addrlen will return a value greater
than was supplied to the call.
If no pending connections are present on the queue, and the socket is not marked as nonblocking, accept() blocks the caller
until a connection is present. If the socket is marked nonblocking and no pending connections are present on the queue, ac‐
cept() fails with the error EAGAIN or EWOULDBLOCK.
In order to be notified of incoming connections on a socket, you can use select(2), poll(2), or epoll(7). A readable event
will be delivered when a new connection is attempted and you may then call accept() to get a socket for that connection.
Alternatively, you can set the socket to deliver SIGIO when activity occurs on a socket; see socket(7) for details.
If flags is 0, then accept4() is the same as accept(). The following values can be bitwise ORed in flags to obtain differ‐
ent behavior:
SOCK_NONBLOCK Set the O_NONBLOCK file status flag on the open file description (see open(2)) referred to by the new file
descriptor. Using this flag saves extra calls to fcntl(2) to achieve the same result.
SOCK_CLOEXEC Set the close-on-exec (FD_CLOEXEC) flag on the new file descriptor. See the description of the O_CLOEXEC
flag in open(2) for reasons why this may be useful.
RETURN VALUE
On success, these system calls return a file descriptor for the accepted socket (a nonnegative integer). On error, -1 is
returned, errno is set to indicate the error, and addrlen is left unchanged.
Error handling
Linux accept() (and accept4()) passes already-pending network errors on the new socket as an error code from accept(). This
behavior differs from other BSD socket implementations. For reliable operation the application should detect the network
errors defined for the protocol after accept() and treat them like EAGAIN by retrying. In the case of TCP/IP, these are
ENETDOWN, EPROTO, ENOPROTOOPT, EHOSTDOWN, ENONET, EHOSTUNREACH, EOPNOTSUPP, and ENETUNREACH.
ERRORS
EAGAIN or EWOULDBLOCK
The socket is marked nonblocking and no connections are present to be accepted. POSIX.1-2001 and POSIX.1-2008 allow
either error to be returned for this case, and do not require these constants to have the same value, so a portable
application should check for both possibilities.
EBADF sockfd is not an open file descriptor.
ECONNABORTED
A connection has been aborted.
EFAULT The addr argument is not in a writable part of the user address space.
EINTR The system call was interrupted by a signal that was caught before a valid connection arrived; see signal(7).
EINVAL Socket is not listening for connections, or addrlen is invalid (e.g., is negative).
EINVAL (accept4()) invalid value in flags.
EMFILE The per-process limit on the number of open file descriptors has been reached.
ENFILE The system-wide limit on the total number of open files has been reached.
ENOBUFS, ENOMEM
Not enough free memory. This often means that the memory allocation is limited by the socket buffer limits, not by
the system memory.
ENOTSOCK
The file descriptor sockfd does not refer to a socket.
EOPNOTSUPP
The referenced socket is not of type SOCK_STREAM.
EPERM Firewall rules forbid connection.
EPROTO Protocol error.
In addition, network errors for the new socket and as defined for the protocol may be returned. Various Linux kernels can
return other errors such as ENOSR, ESOCKTNOSUPPORT, EPROTONOSUPPORT, ETIMEDOUT. The value ERESTARTSYS may be seen during a
trace.
VERSIONS
The accept4() system call is available starting with Linux 2.6.28; support in glibc is available starting with glibc 2.10.
STANDARDS
accept(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (accept() first appeared in 4.2BSD).
accept4() is a nonstandard Linux extension.
On Linux, the new socket returned by accept() does not inherit file status flags such as O_NONBLOCK and O_ASYNC from the
listening socket. This behavior differs from the canonical BSD sockets implementation. Portable programs should not rely
on inheritance or noninheritance of file status flags and always explicitly set all required flags on the socket returned
from accept().
NOTES
There may not always be a connection waiting after a SIGIO is delivered or select(2), poll(2), or epoll(7) return a read‐
ability event because the connection might have been removed by an asynchronous network error or another thread before ac‐
cept() is called. If this happens, then the call will block waiting for the next connection to arrive. To ensure that ac‐
cept() never blocks, the passed socket sockfd needs to have the O_NONBLOCK flag set (see socket(7)).
For certain protocols which require an explicit confirmation, such as DECnet, accept() can be thought of as merely dequeuing
the next connection request and not implying confirmation. Confirmation can be implied by a normal read or write on the new
file descriptor, and rejection can be implied by closing the new socket. Currently, only DECnet has these semantics on
Linux.
The socklen_t type
In the original BSD sockets implementation (and on other older systems) the third argument of accept() was declared as an
int *. A POSIX.1g draft standard wanted to change it into a size_t *C; later POSIX standards and glibc 2.x have socklen_t *
.
EXAMPLES
See bind(2).
SEE ALSO
bind(2), connect(2), listen(2), select(2), socket(2), socket(7)
Linux man-pages 6.03 2022-12-04 accept(2)